ParameterDescriptor should allow optional() declaration

[nebhale]

Currently, the ParameterDescriptor does not allow an optional() descriptor. This makes it difficulty to document parameters that are not required, and do not exist in the current example. The ParameterDescriptor should be able to be declared optional.

[wilkinsona]

I thought this was going to be annoyingly hard to change due to the ParameterDescriptor being shared between path parameters and request parameters as optional path parameters seemed a bit strange. However, it would appear that JAX-RS supports them so it makes sense for REST Docs to be able to document them as I don't want REST Docs to restrict people's API design unnecessarily, however strange I think it may be.

[adamdec]

Let me work on that if you do not mind.

[wilkinsona]

@adamdec How's it going?